Error messages for keys which are disabled present both a warning tone, a displayed message, and an audible message, which varies depending on the context of the error condition. Pressing an invalid key where the selection of a menu option (from the buttons located next to the presentation screen) would be expected presents the message: "The key you pressed is inactive. Please select one of the valid options using the buttons next to the screen." Pressing an invalid key where the entry of a numeric quantity would be expected presents the message: "The key you pressed is inactive. Please enter the [quantity of stamps/dollar amount of postage] you want." A second error condition before the completion of the first will interrupt and preempt the audible message presented by the first error condition.
The cancel key (not pictured in the interface screens, but included next to the keypad below) allows the user to back up to the immediately previous screen in all cases, except while the user is in the middle of a transaction, or the transaction is complete. Here, we use the term transaction not generically to indicate the entire process of dialogue between the user and the system, but specifically the portion of the dialogue where the user is actually inserting something (either payment or a package) into the system. In the cases where the user attempts to cancel during one of these stages, it is assumed that he wishes to halt the entire operation, and is returned to the main menu of the system. If the user selects cancel after the package has been weighed, or after payment has been fully received and stamps/receipt dispensed, the cancel button is disabled (and will generate the inactive key error, as described above).
Schematics indicating the location of various input and output devices for the interface are given at each step to help the user identify the relative position of the system where items should be placed or received. These schematics were not included in the prototype screens because the actual layout of the discrete component devices of the system were omitted from the scope of this project phase. These devices would also be clearly marked on the system chassis.
The keypad layout used for the TURBOMail system is presented below. It has been revised to follow the standard telephone button layout for familiarity, and includes enter, clear, and cancel buttons. The operation of the cancel button is described above. The enter button is used to accept the entry of numerical data, while the clear button simply clears any numbers entered into the system at the current step of the transaction. These buttons are disabled (and therefore generate error messages) if the user is not at a point in the interface where numeric entry is required.
Please note that the interface walkthrough (provided by the allowable actions listed beneath each prototype screen) and the project walkthrough (provided by the [Continue] [Back] [Home] buttons located at the bottom of each page) are separate entities, and that combining navigation through the two may not make sense. In particular, there are screens presented which may not be reachable from the prototype walkthrough; these screens correspond to the action of the cancel button, whose behavior has been described in this document.